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Daniel R Neal, Wei Xiong, Steve Farrer, Thomas D Raymond, James Copland; Corneal and wavefront irregularity maps for determination of ocular abnormality. Invest. Ophthalmol. Vis. Sci. 2014;55(13):4852.
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During the patient work up for refractive surgery, it is important to screen for keratoconus and other conditions prior to refractive surgery treatment. A number of metrics have been used in corneal topography to categorize these conditions and alert the surgeon to possible complications. However, in some cases these metrics are difficult to interpret due to dry eye or measurement instrument inaccuracies. With the advent of diagnostic instruments that can accurately characterize both cornea and whole eye aberrations, new methods of analyzing the data may aid in this categorization. Two new maps have been developed that allow high spatial frequency information to be highlighted. These are the wavefront- irregularity and corneal-irregularity maps. The purpose of this study is to evaluate the efficacy of these maps in determining unusual ocular conditions.
The iDesign combined aberrometer/topographer provides data for both the whole eye aberrations (WF) and the corneal surface (CT) in near rectangular grids at about 0.2 mm resolution. This data was analyzed two ways: 1) high resolution, point-by-point analysis, and 2) low spatial frequency surface fitting through Zernike decomposition (through 6th order). Since the aberrations of the eye (especially low order) often dominate the maps, it is often difficult to determine smaller (but still optically significant) errors from the total aberration or corneal elevation maps. However, by subtracting the Zernike decompositions from the initial surface (for both WF and CT), the irregularity in the associated surface can be determined.
As an example, Patient RR has had a previous RK treatment. There is not really any evidence visible in the wavefront higher order or corneal topography axial power or mean curvature maps (Figure 1). These maps only show the relatively smooth surfaces, dominated by the low order aberrations. However, evidence of radial cuts is evident in both the WF and CT irregularity maps once the slowly varying components have been subtracted (Figure 2).
The irregularity maps (both WF and CT) provide additional information that will help in treatment guidance. They help to distinguish between different types of clinical conditions and help to confirm or eliminate that the tear film is the cause of some of the corneal abnormality.
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